Recently, an optical communication is commonly used, which uses light as an information transmission medium for the purpose of high capacity information transmission and high speed information communication. Also, in these days, it is possible to easily convert a 10 giga bit per second (Gbps) electrical signal into a laser beam by using a semiconductor laser diode chip having an approximately 0.3 mm horizontal length and an approximately 0.3 mm vertical length, and to easily convert an optical signal transmitted through an optical fiber into the electrical signal by using a semiconductor light-receiving device. Light is an energy wave having very peculiar characteristics. In order that several lights existing in an area at the same time interact with one another, the interacting lights should have the same wavelength or the same phase and have the same traveling direction. Therefore, the lights have a very low degree of interference, and a wavelength division multiplexing (WDM) method is now widely used, which transmits the laser beam having various different wavelengths to one optical fiber by using the characteristics of the light. For the application of the WDM method, there is a requirement of a laser beam source capable of emitting laser beam having a wavelength fixed appropriately according to the interval between adjacent wavelengths.
At present, the interval between the wavelengths in the dense wavelength division multiplexing tends to be gradually reduced to 1.6 nm (nano meter), 0.8 nm or 0.4 nm.
Therefore, for such a wavelength division multiplexing, the wavelength width of the light source should be very small, and the wavelength of the laser beam source should be very strictly fixed with respect to the changes of various laser beam source driving environments, such as a temperature, laser driving current, etc. Wavelength precision within ¼ of the wavelength interval is generally required. Therefore, when the wavelength interval of the wavelength multiplexing is 1.6 nm, 0.8 nm or 0.4 nm, the degree of the wavelength stabilization should be precisely controlled within +/−0.2 nm, +/−0.1 nm and +/−0.05 nm.
In the dense wavelength division multiplexing communication, 32-channel wavelength and 64-channel wavelength require mutually different light sources, and thus, it has been difficult to prepare a separate light source corresponding to each wavelength. Therefore, a wavelength-tunable laser has been popular, which is able to respond to all the various channels. The wavelength-tunable laser apparatus has a structure in which the wavelength is tunable. The wavelength-tunable mechanism may cause the instability of the wavelength. As a result, a wavelength measuring method capable of measuring the wavelength of the laser beam is now widely used in the wavelength-tunable laser.
FIG. 1 shows an existing butterfly type laser package having a wavelength stabilization device therewithin cited in registered Korean Patent No. 10-0871011 of the present inventor. As shown in FIG. 1, in the existing butterfly type package having the wavelength measuring device in the laser diode package, the laser beam emitted from one side of a laser diode chip 2 is connected to an optical fiber 9 and is applied to the communication, and the laser beam emitted from the other side of the laser diode chip 2 is collimated and split into two branched beams. Then, a wavelength selective transmission filter having transmission characteristics which are changed according to the wavelength and a monitoring photo diode 6 for monitoring the intensity of the beam which has transmitted through the wavelength selective filter 5 are disposed on the path of one beam. A photo diode 4 for detecting the intensity of the beam emitted from the laser diode chip 2 is disposed on the path of the other beam. The transmittance in which the laser beam transmits through the wavelength selective filter 5 is computed by comparing current flowing through the two disposed photo diodes 6 and 4, and wavelength information of the laser beam is obtained through the transmittance. Therefore, in the existing butterfly type laser package having the wavelength stabilization device therewithin cited in the patent, the optical communication is performed by using the laser beam emitted from the one side of the laser diode chip, and the light power and wavelength of the laser are obtained by using the laser beam emitted from the other side of the laser diode chip.
FIG. 2 shows a general TO can type package for optical communication. In general, the TO can type package has a much lower manufacturing cost and a smaller volume than those of the butterfly type package, and thus, is widely used in the optical module for communication. However, in the TO can type package, since a stem bottom surface on the optical parts are placed is perpendicular to the light beam output direction, it is required that the direction of the beam which is disposed on the stem bottom surface and is emitted in parallel with the stem bottom surface should be changed into a perpendicular direction by using a 45-degree reflection mirror. However, the current optical communication requires that the volume of the optical module is minimized and many optical modules for subscribers are installed in a telephone station having a limited area. Thus, in order to minimize the volume of the wavelength-tunable laser including a wavelength measuring device and to reduce the cost of the wavelength-tunable laser, there is a requirement for a method for mounting a wavelength-tunable laser module including the wavelength measuring device on the TO can type package.
FIG. 3 shows a TO can type package module having a front optical monitoring function disclosed in the Korean Patent No. 10-09136251 registered by the present inventor. As described in the patent, the TO can type package module having the front optical monitoring function shown in FIG. 3 can be applied when the intensities of the laser beams emitted from the one and the other sides of the laser diode chip achieve a constant ratio. It is indicated that, in the laser diode chip of which one side has a reflectance less than 0.1% like a reflective semiconductor optical amplifier (RSOA), since the intensities of the beam emitted to both sides of the laser diode chip are changed depending on the current flowing through the laser diode chip, the optical monitoring at the rear side of the laser diode chip does not reflect the intensity of the laser beam emitted from the front side of the laser diode chip. Therefore, the patent discloses a method for directly monitoring the beam emitted from the front side of the laser diode chip by transmitting a part of the beam emitted from the front side of the laser diode chip (toward the optical fiber) through the 45-degree reflection mirror.
Meanwhile, FIG. 4 shows a method for obtaining the intensity and wavelength information of the laser beam by using the laser beam emitted to the front and rear sides of the laser diode chip in the TO can type package in the Korean Patent No. 10-0871011 registered by the present inventor. However, as described in the Korean Patent No. 10-0913625 registered by the present inventor, the method can be applied to distributed feedback laser diode (DFB-LD) chip and is difficult to apply to the RSOA.
Therefore, like the RSOA used in the wavelength-tunable laser, in the laser diode chip having a very big reflectance difference, for example, one side of the laser diode chip has a reflectance less than 0.1% and the other side has a reflectance greater than 10%, it is not possible to monitor the wavelength and intensity of the laser beam by using the beam emitted from both sides of the laser diode chip.
Therefore, a special means is required so as to manufacture the wavelength-tunable laser including an inexpensive wavelength measuring device for the purpose of utilization of the inexpensiveness and miniaturization of the TO can type package.